The present invention relates to controllable attenuators and attenuation systems for attenuating optical energy transmitted through a fiber optic.
There are requirements in fiber optic systems for precise control of optical signal levels entering various system components. This is particularly true for systems at test and characterization stages of deployment. A controllable optical attenuator can be used, for example, to characterize and optimize the optoelectronic response of high-speed photoreceivers, wherein the detection responsivity is dependent on the average optical power incident on the photodiode.
The majority of controllable fiber optic attenuators currently commercially available rely on thin-film absorption filters, which require breaking the fiber and placing the filters in-line. Controllable attenuation is then achieved by mechanical means such as rotating or sliding the filter to change the optical path length within the absorptive material. This adversely impacts the response speed of the device, the overall mechanical stability, zero attenuation insertion loss and optical back reflection. In general, broken fiber designs suffer numerous disadvantages such as high insertion loss, significant back reflection, and large size. These factors can be minimized, although such corrective measures typically result in added cost and/or size.
Additional issues have impeded the development of thermo-optic variable attenuators, including: (i) the thermal mass of surrounding materials and/or structures which significantly degrades device response time; and (ii) spectrally non-uniform attenuation, resulting from a dispersion mis-match between the optical mode index of the underlying transmission media and a controllable overlay material.
Improved controllable fiber optic attenuators and attenuation systems are therefore required which keep the optical fiber core intact, which achieve controllable attenuation via control of radiative loss from the fiber, and which offer improved response time and spectral uniformity over the wavelength bands of interest.
The shortcomings of the prior approaches are overcome, and additional advantages are provided, by the present invention, which in one aspect relates to an attenuator for attenuating optical energy transmitted through a portion of a fiber optic. The portion of the fiber optic has an exposed surface through which at least some of the optical energy can be controllably extracted. This portion of the fiber optic is suspended within a support structure, and a controllable material is formed over the exposed surface of the fiber optic for controllably extracting the optical energy. The controllable material controllably extracts the energy according to a changeable stimulus, e.g., temperature. The portion of the fiber optic and the controllable material are both positioned to be substantially thermally insulated from any surrounding structures.
The attenuator may also include a controllable heating/cooling source in operative contact with the controllable material to change the temperature thereof, and therefore the attenuating effects thereof. A substantially cylindrical housing may be provided, which includes the support structure, and encloses the portion of the fiber optic, the controllable material and the controllable heating/cooling source. A sensor may also be provided for sensing the temperature of the controllable material, and control leads for both the controllable heating/cooling source and the temperature sensor are provided.
By suspending the fiber optic, and substantially thermally insulating the fiber optic and the controllable material from any surrounding support structures, device size is reduced, and thermo-optic response time is improved.
To improve spectral uniformity of the response of the attenuator across a given wavelength band (e.g., 1520 nm to 1580 nm), the controllable material may have its optical dispersion properties controlled (e.g., matched) in accordance with those of the fiber in this band. Preferably, the controllable material has its optical dispersion properties substantially matched to those of the fiber in the band of interest. The control of the dispersion properties is effected using, for example, polymers with added dyes, discussed in detail in the co-filed Application entitled xe2x80x9cDISPERSION CONTROLLED POLYMERS FOR BROADBAND FIBER OPTIC DEVICES.xe2x80x9d
The present invention, in another aspect, relates to methods for attenuating optical energy in a fiber optic using the attenuator discussed above, as well as methods for forming the attenuator discussed above.
The xe2x80x9cblockless,xe2x80x9d dispersion controlled attenuator of the present invention provides a high performance design with wide flexibility. The simplicity of the design permits low-cost, high-volume manufacturing without sacrificing optical performance.